A motion transmission system in a bicycle comprises a chain extending between toothed wheels associated with the axle of the pedal cranks and with the hub of the rear wheel. When there is an assembly of toothed wheels comprising more than one toothed wheel at at least one of the axle of the pedal cranks and the hub of the rear wheel, and the motion transmission system is therefore provided with a gearshift, a front derailleur and/or a rear derailleur are provided for.
In case of an electronic gearshift, each derailleur of the transmission system comprises a guide element—also known as chain guide or cage or, in case of a rear derailleur, rocker arm—movable to displace the chain among the toothed wheels, and an electromechanical actuator to displace the chain guide.
The actuator typically comprises a motor, typically an electric motor, coupled with the chain guide through a linkage, such as an articulated parallelogram, a rack system or a worm screw system. Typically, the electric motor is provided with a gear reduction mechanism. The assembly of electric motor and gear reduction mechanism is referred to hereinafter as motor gear (or geared motor). The actuator typically further comprises a sensor or transducer of the position, speed, acceleration and/or direction of rotation of the rotor or of any moving part downstream of the rotor, down to the chain guide itself.
Control electronics changes the gear ratio automatically, for example based on one or more detected variables, such as the travel speed, the cadence of rotation of the pedal cranks, the torque applied to the pedal cranks, the slope of the travel terrain, the heart rate of the cyclist and similar, and/or the gear ratio is changed based on commands manually input by the cyclist through suitable control members, for example levers and/or buttons, typically provided on one or two manual command devices mounted to the bicycle.
The motor gear of the derailleur is sized to withstand the external stresses, including the force exerted on it by the tension of the chain. During use of the bicycle, the vibrations, impacts and/or other stresses of the chain and/or of the chain guide can apply an excessive force on the motor gear. If such a force is not suitably counteracted, it can cause the involuntary or “passive” displacement of the motor gear, if not actually damage thereof. An involuntary displacement of the motor gear and of the chain causes greater friction with the engaged toothed wheels, and possibly even causes the chain to fall or an involuntary change of the gear ratio. Furthermore, it can cause problems during subsequent gearshifting due to the misalignment between what the control electronics assumes to be the position of the derailleur and the actual position. On the other hand, an oversizing of the motor gear is disadvantageous in terms of bulk, weight and cost.
EP 2 727 810 A2 discloses an electric derailleur motor unit with a self-lock function, that includes: a base member; a motor mounted to the base member, the motor having a motor shaft rotatable about a first rotational axis; an output shaft rotatable about a second rotational axis; and a drive train operatively disposed between the motor shaft of the motor and the output shaft, the drive train including an anti-reverse clutch, the anti-reverse clutch being configured to transmit rotation of the motor shaft of the motor in both rotational directions about the first rotational axis to the output shaft, and the anti-reverse clutch being further configured to prevent the output shaft from rotating in both rotational directions about the second rotational axis while the output shaft receives an external rotational torque from outside of said electric derailleur motor unit.
More in particular, the anti-reverse clutch includes a clutch case mounted to the base member, an input member rotatable with respect to the clutch case, the input member being operatively coupled to the motor shaft of the motor, an output member rotatable with respect to the clutch case, the output member being operatively coupled to the output shaft, and an engagement element operatively disposed between the clutch case and the output member.
In the case of use, within said EP 2 727 810 A2, of an anti-reverse clutch according to JP 2713601, the engagement element has a plurality of rollers radially disposed between the clutch case and the output member. The rollers are arranged to lock the output member with respect to the clutch case in both rotational directions about a rotational axis of the output member unless there is drive from the input member. On the other hand, the rollers are arranged to release the output member with respect to the clutch case when there is drive from the input member in either rotational direction about a rotational axis of the input member. Furthermore, if the input member rotates further, then the input member is directly connected to the output member to transmit rotation of the input member to the output member.
As an alternative, EP 2 727 810 A2 provides for using an anti-reverse clutch wherein, when an input member of the anti-reverse clutch is rotated, then an output member of the anti-reverse clutch also rotates; on the other hand, if an external rotational torque from outside the anti-reverse clutch in both rotational directions is exerted to the output member, then the output member can freely rotate with respect to the input member, and the rotational torque is not transmitted from the output member to the input member.
EP 2 727 810 A2 further discloses bicycle derailleur motor gear unit configured to reduce motor speed of a motor and transmit motor torque of the motor to a derailleur, the bicycle derailleur motor gear unit comprising: an output shaft; a fixed sun gear; an output sun gear rotatable relative to the fixed sun gear, the output sun gear being fixedly coupled to the output shaft; a plurality of planet gears; and a carrier rotatably and revolvably supporting the planet gears; each of the planet gears having a first gear that engages with the fixed sun gear and a second gear that engages with the output sun gear, the first gear and the second gear being coaxially coupled to each other; the fixed sun gear, the output sun gear, the first gear, and the second gear having tooth counts, respectively, such that the fixed sun gear, the output sun gear, the first gear, and the second gear lock while an external force is exerted to the output sun gear. Therefore, the rotation of the motor responsive to the operation of a switch is transmitted from the output shaft to the chain guide to perform a shift change, but when the force from the chain is added to the chain guide, the planetary gear reduction mechanism self-locks, stopping rotation and takes the stress. Consequently, the chain guide is retained at its original position, and an unintended shift change does not occur.
With regard to this second solution of such a document, the Applicant observes that the motor gear has a low efficiency, due to the large number of toothed couplings. Furthermore, the required relationship between the number of teeth of the various gears greatly limits the freedom to design the motor gear.
With regard to the first solution of such a document, the Applicant observes that an anti-reverse clutch like the one described above has a quite complex construction and in which the sizing is very critical; furthermore, such a clutch is delicate due to the return elements housed within the clutch case.
The technical problem underlying the present invention is to provide a derailleur that is not subject to involuntary displacement without imposing particular design constraints on the motor gear of the derailleur.